JP3299859B2 - RF modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RF modulator used for a VTR (Video Tape Recorder), a home game machine, or the like.

[0002]

2. Description of the Related Art An RF (Radio Frequency) modulator is a device for converting a baseband signal including a video signal and an audio signal into an RF signal broadcast from a broadcasting station.

For example, when a recorded video tape is reproduced using a VTR, a baseband video signal and an audio signal are output. If there is a television monitor having an input terminal for a baseband signal, it can be easily watched by connecting the cable as it is.

However, in a television receiver having no input terminal for a baseband signal, the baseband signal is frequency-converted so as to match a channel band broadcast from an existing television broadcasting station, and converted into an RF signal. It is necessary to input from the antenna terminal.

[0005] RF modulators are typically designed to selectively output one of two channel bands. This is because when one channel band is already used as a broadcast wave, the other channel band is selected to prevent interference with the broadcast wave and to prevent adverse effects on video and audio.

For example, in a VTR set used in Japan, the output band of the RF modulator is set to 1ch (channel) and 2ch in many cases. For example, if 1ch is already used as a broadcast wave at the installation location of the device, 2ch is selected as the output band of the RF modulator.

FIG. 4 shows 1ch and 2ch in Japan.
5 is a graph showing a broadcast frequency band of the present invention. In the frequency band of 1ch, the frequency of the video carrier P1 is 91.25M
Hz and the frequency of the voice carrier S1 is 95.75 MHz. In the 2ch frequency band, the video carrier P
2 is 97.25 MHz, and the frequency of the voice carrier S2 is 101.75 MHz. Since 6 MHz is secured in one channel band, 1ch and 2ch do not overlap, and it is theoretically impossible to interfere with each other. In addition, existing broadcasting stations are strictly regulated so as not to emit radio waves outside a predetermined channel band.

On the other hand, since the signal level output from the RF modulator is weak, in order to prioritize simplification of the circuit configuration, extra frequency components other than the target frequency are output within a range that does not affect other devices. May be.

FIG. 5 is a circuit diagram showing an example of a conventional RF modulator. The modulation circuit 10 has a frequency of 91.25 MHz.
Or a local oscillator that oscillates at 97.25 MHz and a mixer that performs frequency conversion. When a baseband signal including a video signal and an audio signal is input,
An RF signal in a channel band of 1ch or 2ch is output. The frequency switching circuit 11 switches the oscillation frequency of the local oscillator and selects a channel band of the RF signal. The RF signal output from the modulation circuit 10 is output from the output terminal 13 to a television receiver (not shown) or the like via the DC component cutting capacitor C3 and the level adjusting resistor R1. Note that a coil L2 for impedance matching and a capacitor C2 are connected in parallel between the output terminal 13 and the ground.

FIG. 6 is a graph showing a conventional frequency characteristic. The RF signal from the modulation circuit 10 is supplied to the capacitor C3
And the signal is output as it is via the resistor R1,
The whole frequency characteristic becomes almost flat, and 1ch and 2c
There is not much attenuation in the channel band of h.

[0011]

In the conventional RF modulator, since the number of parts and the cost are reduced as much as possible, a frequency component that interferes with another channel band is output. For example, when the output band of the RF modulator is selected to be 2ch, as shown in FIG.
2 and the voice carrier S2 are mainly output, but the mixer of the modulation circuit 10 also generates both lower side waves S2 '. The frequency of the lower side wave S2 ′ is 97.25− (101.75).
−97.25) = 92.75 MHz, which appears in the band of one channel. Here, if 1ch is used as a broadcast wave, interference with the lower side wave S1 'occurs, which has a great adverse effect on the video. Therefore, while the RF modulator is operating, the 1-channel television broadcast cannot be watched.

In addition, in the circuit configuration shown in FIG. 5, it is necessary to add a very complicated and expensive circuit in order not to output the lower side wave S1 '.

The output band of the RF modulator is 1c
h, the frequency of the disturbance is 9
1.25- (95.75-91.25) = 86.75M
Hz, and does not affect any broadcast channels.

An object of the present invention is to provide an RF modulator capable of greatly attenuating an interference wave level with a simple circuit configuration.

[0015]

According to the present invention, there is provided an RF circuit comprising a modulation circuit for biasing a baseband signal to an RF signal of one or two channel bands of television broadcasting.
In the modulator, a transistor is inserted in series with the output line of the modulation circuit, and a transistor is connected so that an input side is a collector and an output side is an emitter, and a first coil and a first coil connected in parallel between the collector and the emitter.
A parallel resonance circuit consisting of a capacitor, connected between the emitter and ground, to reduce the emitter potential in the DC region to 0V
And a second coil having a function of performing the function of controlling the base current of the transistor by an external frequency switching signal, whereby the transistor is turned on or off, and the resonance frequency of the parallel resonance circuit is one channel band and two. When one channel band, which is a low frequency in the channel band, is set to be attenuated and an RF signal of the one channel band is output, the transistor is turned on, and an RF signal of the two channel band, which is a high frequency, is output. An RF modulator in which a transistor is turned off in such a case.

[0016]

According to the present invention, the overall filter characteristics can be controlled by interposing a parallel resonance circuit on the signal output line and changing the characteristics of the parallel resonance circuit by turning on or off the transistors. Therefore, when one channel band is selected and an interference wave appears in the other channel band, the parallel resonance circuit can be operated to greatly reduce the interference wave level.

The resonance frequency of the parallel resonance circuit is set so as to attenuate the lower one channel band of the one channel band and the two channel band of the television broadcast, and an RF signal of the one channel band is output. In this case, the transistor is turned on, and when outputting a high frequency 2-channel band RF signal, the transistor is turned off, so that the frequency of the audio carrier is set higher than that of the video carrier. Below, one parallel resonance circuit for attenuation of the interference wave is sufficient.

[0018]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. The modulation circuit 20 operates at 91.25 MHz or 97.2 MHz.
It is composed of a local oscillator oscillating at 5 MHz, a mixer for frequency conversion, and the like. When a baseband signal including a video signal and an audio signal is input, 1ch or 2
The RF signal of the channel band of ch is output. The frequency switching circuit 21 switches the oscillation frequency of the local oscillator,
Select the channel band of the signal.

In an output line between the input terminal 22 and the output terminal 23, a DC component cutting capacitor C3 and a level adjusting resistor R1 are connected in series to the input terminal 22, and the input terminal 22 side is a collector and an output terminal. 23
Transistor Q1 connected so that its side becomes an emitter
Is inserted.

A parallel resonance circuit 25 including a coil L1 and a capacitor C1 is connected in parallel between the collector and the emitter of the transistor Q1. The resonance frequency of the parallel resonance circuit 25 is set so as to attenuate the 1ch band which is a low frequency channel band.

A coil L2 and a capacitor C2 are connected in parallel between the emitter and the ground, and are matched to have an impedance of, for example, 75Ω. In particular, the coil L2 has a function of setting the emitter potential in the DC region to 0V.

A resistor R2 for determining a base current is connected to the base of the transistor Q1, and a frequency switching signal from a frequency switching circuit 21 is input via a control terminal 24.

Next, the operation will be described. When outputting the RF signal of the 1ch band, the frequency switching circuit 21 outputs the frequency switching signal for 1ch, sets the local oscillation frequency of the modulation circuit 20 to 91.25 MHz, and sets the control terminal 24 to about 2 to 5 V. , A predetermined base current is caused to flow, and the transistor Q1 is turned on.

FIG. 2 is a graph showing frequency characteristics when the transistor Q1 is conducting. Transistor Q1
Is in a conductive state, the RF signal passes through the transistor Q1 as it is, so that the parallel resonance circuit 25 including the coil L1 and the capacitor C1 does not function, and is not much different from the conventional one shown in FIG. It becomes flat and the channel bands of 1ch and 2ch are not attenuated much.

Therefore, the 1-channel RF signal output from the modulation circuit 20 passes through the capacitor C3, the resistor R1, and the transistor Q1, and is not greatly attenuated from the output terminal 23 to a television receiver (not shown). Output to

Next, when outputting an RF signal in the 2ch band, the frequency switching circuit 21 outputs a frequency switching signal for 2ch, and the local oscillation frequency of the modulation circuit 20 is changed to 97.2.
The frequency is set to 5 MHz and a frequency switching signal is output so that the control terminal 24 is at the ground potential (0 V). Then, the base current becomes 0 and the transistor Q1 is turned off.

FIG. 3 is a graph showing frequency characteristics when the transistor Q1 is turned off. Transistor Q1
In the cut-off state, the RF signal changes to a filter characteristic that greatly attenuates the 1ch band around the resonance frequency by the function of the parallel resonance circuit 25 including the coil L1 and the capacitor C1. On the other hand, the 2ch band is not so attenuated.

Therefore, the 2ch RF signal output from the modulation circuit 20 passes through the capacitor C3, the resistor R1 and the parallel resonance circuit 25 and is not greatly attenuated from the output terminal 23 to the television receiver (not shown). ) Etc. are output. On the other hand, the lower side wave S2 'serving as an interfering wave is trapped by the parallel resonance circuit 25 and greatly attenuated, so that the output level to the outside is suppressed.

The measurement conditions under which the characteristics shown in FIGS. 2, 3 and 6 are obtained depend on the integrated circuit CXA112 as the modulation circuit 20.
2AM (Sony), transistor Q1 is 2S
C2735 (manufactured by Hitachi), coil L1 has a wire diameter of 0.45 m
m, the winding diameter is 2.0 mm, the number of windings is three, and the coil L2 has a wire diameter of 0.3 mm
4mm, winding diameter 2.5mm, number of turns 5 times, capacitor C1,
C2 and C3 are 180 pF, 39 pF and 1000, respectively.
pF and resistors R1 and R2 are 100Ω and 1.5kΩ, respectively.
Is used.

In the above embodiment, an example has been shown in which the 1ch and 2ch bands of television broadcasting are used as the first and second channel bands selectively output by the RF modulator. Any two of the above may be used, and VHF, UHF, S
Any channel band such as HF may be used.

[0031]

As described in detail above, according to the present invention, the overall filter characteristics can be improved by interposing a parallel resonance circuit in the signal output line and changing the characteristics of the parallel resonance circuit by turning on or off the transistors. Can be controlled. Therefore, when one channel band is selected and an interference wave appears in the other channel band, the parallel resonance circuit can be operated to greatly reduce the interference wave level.

Thus, an RF modulator that can greatly attenuate the level of an interference wave with a simple circuit configuration is realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a graph showing frequency characteristics in a conductive state of a transistor Q1.

FIG. 3 is a graph showing frequency characteristics in a cutoff state of a transistor Q1.

FIG. 4 is a graph showing broadcast frequency bands of 1ch and 2ch in Japan.

FIG. 5 is a circuit diagram showing an example of a conventional RF modulator.

FIG. 6 is a graph showing a conventional frequency characteristic.

[Explanation of symbols]

 Reference Signs List 20 modulation circuit 21 frequency switching circuit 22 input terminal 23 output terminal 24 control terminal 25 parallel resonance circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03D 7/00 H04N 5/38

Claims (1)

(57) [Claims] 1. An RF modulator comprising a modulation circuit for biasing a baseband signal to an RF signal in one or two channel bands of television broadcasting, wherein the input side is inserted in series with an output line of the modulation circuit. A transistor connected so that the collector and the output side serve as an emitter; a parallel resonance circuit including a first coil and a first capacitor connected in parallel between the collector and the emitter; a DC region connected between the emitter and the ground Emi in
A second coil having a function of reducing the potential of the transistor to 0 V. When a frequency switching signal from the outside controls the base current of the transistor, the transistor is turned on or off, and the resonance frequency of the parallel resonance circuit is The first channel band is set so as to attenuate the lower frequency band of the one-channel band and the two-channel band. When outputting the RF signal of the one-channel band, the transistor is turned on and the two-channel band of the higher frequency is outputted. Wherein the transistor is turned off when the RF signal is output.